The hard signal: E.164-normalised phone number.
Every phone number β whether the nominator typed +233244123456, 0244123456, or 244123456 β is normalised to the same E.164 string before it hits the database. Hard matches are exact and surfaced in red badges immediately.
You can tune the hard-match behaviour per event: block duplicate phone submissions outright when maxNominationsPerPhone is 1 per category, or allow them and just flag with a warning. For most awards the block is the right default β it prevents the same nominator from stuffing the pool on behalf of their preferred candidate.
The dedupe check also runs at public submission time. If a nominator re-submits the same phone, the form shows a soft warning without leaking any nominator PII β the endpoint returns only { exists: boolean }. They can still submit if they genuinely believe it is a different person.